My husband is a British citizen, and when he moved here, he received his social security card with the same notation on it.
It doesn't mean that the number is "valid for work only"... it means that he must have DHS (Department of Homeland Security) authorization to get a job (for which he would need that number).
Once he was here, he had to apply for a "Work Permit", but got his social security number before that. Thus the notation on the card, which basically means "Here is your social security number, but you can't use it to work unless you have work authorization from immigration".
Now that he's a permanent resident, that number can be converted to a "permanent" number and a new card will be issued without the disclaimer.
Hope that's helpful!
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